Photomasking

ABSTRACT

An attenuated phase-shifting photomask (APSM) provides adjustment of attenuation from mask to mask. The APSM includes a multilayer substrate, a buffer thin film coupled to the substrate, and a top layer thin film on top of the buffer thin film. The thin films are etched with a circuit pattern to form a photomask, and are chosen to have certain thicknesses which would provide adjustment of attenuation within a specified attenuation operating range and appropriate phase shift without changing said buffer thin film and said top layer thin film.

BACKGROUND

This application is a divisional of U.S. patent application Ser. No.10/389,465, filed on Mar. 13, 2003 now U.S. Pat. No. 6,818,361, which isa continuation of U.S. application Ser. No. 09/430,689, filed on Oct.29, 1999, now U.S. Pat. No. 6,562,522; the disclosure of the priorapplication is considered part of (and is incorporated by reference in)the disclosure of this application.

This disclosure relates to photomasking.

Optical lithography is a technology used to print patterns that defineintegrated circuits onto semiconductor wafers. Typically, a pattern onan attenuation photomask is imaged by a highly accurate camera. Theimage is transferred onto a silicon wafer coated with photoresist.Continued improvements in optical lithography have enabled the printingof ever-finer features. This has allowed the integrated circuit industryto produce more powerful and cost-effective semiconductor devices.

A conventional binary mask that controls the amplitude of light incidentupon a wafer is often inadequate when the integrated circuit (IC)feature size is small.

Phase-shifting masks can be used for optical lithography for thegeneration of IC feature sizes below one micron such as 0.25 micron.Under these subwavelength conditions, optical distortions as well asdiffusion and loading effects of photosensitive resist and etchprocesses cause printed line edges to vary. Phase shifting improves theresolution that optical lithography can attain, producing smaller,higher-performance IC features by modulating the projected light at themask level.

Successors to optical lithography are being developed to further improvethe resolution. Extreme-ultraviolet (EUV) lithography is one of theleading successors to optical lithography. It may be viewed as a naturalextension, since it uses short wavelength optical radiation to carry outprojection imaging. However, EUV lithography (EUVL) technology isdifferent from the technology of optical lithography in that theproperties of materials with respect to EUV radiation are different fromtheir properties with respect to visible and UV ranges. For example, theEUV radiation is strongly absorbed in virtually all materials, includinggas. Thus, EUVL imaging systems often utilize entirely reflectiveoptical elements rather than refractive elements, such as lenses.

SUMMARY

A photomasking technique provides adjustment of attenuation from mask tomask within a specified attenuation operating range and appropriatephase shift without changing the material of thin film layers disposedon a substrate.

The photomask includes a substrate, a first film coupled to thesubstrate, and an upper layer film on top of the first film. The firstfilm has a first thickness of a particular material. The upper layerfilm has a second thickness of another material. The films are etchedwith a circuit pattern to form a photomask, and are chosen to havecertain thicknesses which would provide adjustment of attenuation withina specified attenuation operating range and appropriate phase shiftwithout changing the material of the first film and the upper layer thinfilm. In one embodiment, the specified attenuation operating range isabout 6% to 20% attenuation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will be described in reference to theaccompanying drawings wherein:

FIGS. 1A through 1E show a fabrication process flow for a conventionalbinary photomask;

FIGS. 2A through 2E show a fabrication process flow for anextreme-ultraviolet (EUV) attenuated phase-shifting mask (APSM)according to an embodiment of the invention;

FIGS. 3A through 3F show a process flow for the inspection and repair ofthe EUV APSM according to an embodiment of the invention; and

FIG. 4 shows a process flow diagram for fabricating anintegrated-circuit wafer according to an embodiment of the invention.

DETAILED DESCRIPTION

A conventional binary photomask can be made with a layer of chrome 100deposited on a quartz or glass substrate 102 as shown in FIG. 1A. Thechrome is covered with a photosensitive resist 104 (FIG. 1B). A beamwriter then writes the circuit pattern onto the photomask by exposingthe photosensitive resist with an electron beam or laser, which changesthe molecular composition of the resist 104. During the developingprocess, any positive resist that has been exposed 106 will be removedas shown in FIG. 1C. In an alternative embodiment, the process can beperformed with a negative resist, which removes the non-exposed area.Next, the mask is etched in a process that removes the chrome 100 fromthe area where the resist has been removed 106 (FIG. 1D). Finally, FIG.1E shows all the resist removed from the photomask 108.

The photomask 108, shown in FIG. 1E, is used in much the same way that aphotographic positive is used to make a photograph. A beam of lightprojects the image patterned on the photomask 108, several times, sideby side, onto small areas called “dice” on a silicon wafer. Areas wherethe chrome has been removed pass the beam of light 110 through withoutany attenuation. The remaining areas attenuate the light 112 and createdark spots on the wafer.

An attenuated phase-shifting mask (APSM), which combines attenuationwith a phase-shifting technique, is fabricated by replacing the chromeabsorber layer 100 with a synthesized material. The material issynthesized at a given thickness to meet both phase shift andattenuation requirements. The patterning of such an APSM mask is similarto that of a conventional optical mask, but uses a different absorbermaterial. This difference requires a different etch process. Theachievable attenuation range is very narrow because of the limitation ofavailable material. For example, it is difficult to achieve attenuationon the order of about 10 to 15%, which offers better performance forcertain mask layers than the case of traditional 6% attenuation.Therefore, a change of attenuation for different mask layers means achange of absorber material, and hence, an imposition of new processdevelopment which involves new film selection, deposition, etch, andcleaning parameters for each layer of absorber material.

An APSM process for extreme-ultraviolet (EUV) lithography involveschoosing a single material, for use at an EUV wavelength (13.4 nm),which matches both the phase-shift and the attenuation requirements.This task is difficult to achieve because a different range ofattenuation is often required for different applications. The difficultyis compounded by the high absorption characteristic of condensedmaterial at 13.4 nm, and very little difference in the refractive indexbetween the absorber and vacuum, which determines the phase differencebetween the attenuated region and the reflective region.

FIGS. 2A through 2E show an improved fabrication process flow for an EUVAPSM. The process starts with a multi-layer substrate 200 (FIG. 2A). Insome embodiments, the substrate includes silicon, ultra-low expansionmaterial, etc. This is followed by the deposition of first 202 andsecond 204 thin film layers (FIG. 2B). In some embodiments, the secondthin film layer 204 comprises at least one layer of thin film disposedon top of the first thin film layer 202. FIGS. 2C through 2E illustratethe rest of the process, which includes resist coating and patterning(FIG. 2C), consecutive etching of the top and bottom thin-film layers(FIG. 2D), and stripping of the resist (FIG. 2E). The resultant APSMprovides appropriate attenuation and 180-degree phase shift where thethin-film layers remain 206 and no attenuation and zero phase-shiftwhere the layers have been stripped 208.

In one embodiment, an inspection and repair process follows the toplayer etching. The process flow for the inspection and repair of the EUVAPSM is shown in FIGS. 3A through 3F. The process starts with amulti-layer substrate 200, as before. The first thin-film layer 202,which acts as a buffer layer, is then deposited as shown in FIG. 3A.This is followed by the deposition of the second layer film 204 (FIG.3B) and the resist coating and patterning (FIG. 3C).

The repair process starts after the top layer 204 thin film etching andresist removal has been completed, as shown in FIG. 3D. The top layer204 has been etched using the bottom layer 202 as the etch stop bufferlayer. Once the top layer etching is done, the thin film is inspectedfor defects. FIG. 3D shows two defects 300, 302 typically found duringinspection. Filling the gap 304 with same material as the top layer filmrepairs the defect 300. Chipping away the leftover blob repairs thedefect 302 (FIG. 3E). Finally the bottom layer is etched to produce anEUV APSM as shown in FIG. 3F.

The selection of the type and thickness for each thin-film layer 202,204 should satisfy certain conditions as follows:I≅I _(o)|exp [−(2π/λ)(2α₁ d ₁+2α₂ d ₂)]|²,  (1)|(2π/λ)(2Δn ₁ d ₁+2Δn ₂ d ₂)|=π,  (2)where

I_(o) is the incident light intensity,

I is the light intensity reflected from the absorber region (round-tripreflection),

λ is the wavelength of the light,

α₁ and α₂ are absorption coefficients of films 1 and 2, respectively,

d₁ and d₂ are thicknesses of films 1 and 2, respectively,

n₁ and n₂ are indices of refraction (real part) of films 1 and 2,respectively, for small incident angle, θ, which is an angle between theincident light and the normal to the surface. For larger θ, n1 and n2must take into consideration cos (θ) contributions.

The EUV mask attenuation for a given thickness of a given film isgoverned by the absorption coefficient α and the film thickness d. Theattenuation of the beam in the absorber region after a round-tripreflection from the multi-layer substrate 200 is, in the first orderapproximation, given by equation (1). Both the film thickness and thereal part of the refractive index difference between the absorbermaterial and vacuum govern the phase shift of an absorber. The reflectedlight from thin film to thin film and thin film to vacuum interfaces (onthe order of about 0.01% to 0.02%) are ignored in equation (1) becausethe real parts of the refractive indices are closely matched at EUVwavelength.

Since an APSM with a π phase shift satisfies equation (2), equations (1)and (2) can be solved for d₁ and d₂ as follows:

$\begin{matrix}{{d_{1} = {\left\lbrack {{{- {\lambda\left( {1 - n_{1}} \right)}}{{\ln\left( \frac{I}{I_{o}} \right)}/\left( {8\pi} \right)}} + \frac{{\lambda\alpha}_{1}}{4}} \right\rbrack/\left\lbrack {{\left( {1 - n_{1}} \right)\alpha_{2}} - {\left( {1 - n_{2}} \right)\alpha_{1}}} \right\rbrack}},} & (3) \\{d_{2} = {\left\lbrack {{{- {\lambda\left( {1 - n_{2}} \right)}}{{\ln\left( \frac{I}{I_{o}} \right)}/\left( {8\pi} \right)}} + \frac{{\lambda\alpha}_{2}}{4}} \right\rbrack/{\left\lbrack {{\left( {1 - n_{2}} \right)\alpha_{1}} - {\left( {1 - n_{1}} \right)\alpha_{2}}} \right\rbrack.}}} & (4)\end{matrix}$Therefore, any combination of two films with index of refraction thatsatisfies equations (3) and (4) can be used. The value of d₁ or d₂ mustbe a positive number. In practice, however, the film selection alsoneeds to consider the process compatibility, etch selectivity of the twofilms, and the proper film thickness ratio. The film thickness ratioshould be configured to satisfy the minimum repair buffer layerthickness.

Examples of the two thin film layer selection are shown in Table 1 belowfor an attenuation operating range of 6% to 15%. The desired attenuationfrom about 10% to 15% can be achieved by adjusting the thickness of thetwo film layers without changing the material of each film. The abilityto adjust attenuation without a change in the thin film type enables thesame deposition and etch processes in the manufacturing of the thin-filmlayers. Only the deposition and etching time need to be adjusted toaccommodate the film thickness change.

Buffer Layer Top Layer Attenuation Buffer Layer Thickness Top LayerThickness % Type (d₁) [in Å] Type (d₂) [in Å] 6 SiO₂ 854 Ti 373 7 SiO₂564 Ti 506 7 SiO₂ 766 TiN 304 8 SiO₂ 312 Ti 621 8 SiO₂ 560 TiN 372 9SiO₂ 378 TiN 433 10 SiO₂ 218 TiN 488 10 SiO₂ 1082 Mo 167 11 SiO₂ 1022 Mo184 12 SiO₂ 968 Mo 200 13 SiO₂ 918 Mo 214 14 SiO₂ 872 Mo 227 15 SiO₂ 828Mo 239

Some of the advantages of the EUV APSM and the fabrication process arethe ability to adjust the desired attenuation without changing themanufacturing process of the thin film layers in the EUV APSM and theconsistency of the two-layer absorber process with the currentstate-of-art EUV mask process flow. Further, the process is consistentwith the current state-of-art EUV mask process flow, as well as repairstrategy, that comprises a buffer layer and a metal layer with a bufferlayer being used to absorb the repair damage.

FIG. 4 shows a process flow diagram for fabricating anintegrated-circuit wafer. The process begins by providing a substrate,at step 400. First and second thin film layers are then deposited on topof the substrate (step 402). The first and second thin film layers havefirst and second thicknesses determined by equations (3) and (4),respectively. At step 404, a photosensitive resist is deposited on topof the conducting material. Next, the resist is exposed and developedwith a pattern on an EUV APSM, at step 406. The APSM used in this stepis fabricated using the above-described method. Finally, the conductingmaterial is etched (step 408) and the resist is stripped away (step410). If additional layer is to be patterned, the process is repeated atstep 402.

An apparatus that execute above-specified process is also disclosed. Theapparatus comprises a machine-readable storage medium having executableinstructions that enable the machine to perform the steps in thespecified process.

Other embodiments are within the scope of the following claims. Forexample, the fabrication process for the EUV APSM described above canalso be used for an optical APSM. In this case, equations (3) and (4)need to be modified to include interface reflections due to non-matchedreal part of the reflective index at optical wavelengths.

1. An article comprising a machine-readable medium storing instructionsoperable to cause one or more machines to perform operations comprising:receiving information indicative of one or more optical characteristicsof a particular material; receiving information indicative of one ormore optical characteristics of another material; receiving informationindicative of a desired attenuation amount; receiving informationindicative of a desired phase amount; and using the informationindicative of the one or more optical characteristics of the particularmaterial and the another material, the information indicative of thedesired attenuation amount, and the information indicative of thedesired phase amount for determining a first thickness of the particularmaterial and a second thickness of the another material to attenuatelight of a particular wavelength by the desired attenuation amount asthe light is transmitted through the particular material and the anothermaterial in a forward direction and in a reflected direction, the firstthickness of the particular material and the second thickness of theanother material to shift a phase of the light by the desired phaseamount as the light is transmitted through the particular material andthe another material in the forward direction and in the reflecteddirection.
 2. The article of claim 1, wherein the another materialcomprises tungsten, and wherein the desired attenuation is between about6% and about 10%.
 3. The article of claim 1, wherein the anothermaterial comprises molybdenum, and wherein the desired attenuation isbetween about 10% and about 15%.
 4. The article of claim 1, wherein theinformation indicative of one or more optical characteristics of theparticular material comprises an absorption coefficient of theparticular material and an index of refraction of the particularmaterial, wherein the information indicative of one or more opticalcharacteristics of the another material comprises an absorptioncoefficient of the another material and an index of refraction of theanother material, and wherein the first thickness is represented by d₁,the second thickness is represented by d₂, the particular wavelength isrepresented by λ, the absorption coefficient of the material isrepresented by α₁, the absorption coefficient of the another material isrepresented by α₂, the index of refraction of the material isrepresented by n₁, the index of refraction of the another material isrepresented by n₂, and the ratio of an initial intensity of the light toan exiting intensity of the light is represented by I/I₀, and wherein d₁and d₂ are about equal to: $\begin{matrix}{{d_{1} = {\left\lbrack {{{- {\lambda\left( {1 - n_{1}} \right)}}{{\ln\left( \frac{I}{I_{o}} \right)}/\left( {8\pi} \right)}} + \frac{{\lambda\alpha}_{1}}{4}} \right\rbrack/\left\lbrack {{\left( {1 - n_{1}} \right)\alpha_{2}} - {\left( {1 - n_{2}} \right)\alpha_{1}}} \right\rbrack}},} \\{d_{2} = {\left\lbrack {{{- {\lambda\left( {1 - n_{2}} \right)}}{{\ln\left( \frac{I}{I_{o}} \right)}/\left( {8\pi} \right)}} + \frac{{\lambda\alpha}_{2}}{4}} \right\rbrack/{\left\lbrack {{\left( {1 - n_{2}} \right)\alpha_{1}} - {\left( {1 - n_{1}} \right)\alpha_{2}}} \right\rbrack.}}}\end{matrix}$
 5. The article of claim 1, wherein the particularwavelength is an extreme ultraviolet light wavelength.
 6. The article ofclaim 1, further comprising: receiving information indicative of adifferent desired attenuation amount; and determining a different firstthickness of the particular material using the information indicative ofthe one or more optical characteristics of the particular material andthe another material, the information indicative of the differentdesired attention amount, and information indicative of at least one ofthe desired phase amount and a different phase amount.
 7. The article ofclaim 1, further comprising: receiving information indicative of adifferent desired phase amount; and determining a different firstthickness of the particular material using the information indicative ofthe one or more optical characteristics of the particular material andthe another material, the information indicative of the differentdesired phase amount, and information indicative of at least one of thedesired attenuation amount and a different attenuation amount.
 8. Thearticle of claim 1, further comprising: receiving information indicativeof a different desired attenuation amount and a different desired phaseamount; and determining a different first thickness of the particularmaterial using the information indicative of the one or more opticalcharacteristics of the particular material and the another material, theinformation indicative of the different desired phase amount, and theinformation indicative of the different attenuation amount.